This disclosure is directed to dynamically controllable foil systems, and to methods for controlling such systems. Applications include, but are not limited to, dynamically controlled foil and hydrofoil systems configured to position and maintain spacing between seismic sources and other elements of a marine seismic array.
Seismic arrays with sources and streamers are used to study rock strata and other structures below the surface, for example, as described in U.S. Patent Publication No. 2014/0247691, which is incorporated by reference herein in its entirety for all purposes. One or more marine vessels are typically used to tow the source and/or receiver arrays, in order to obtain relevant geological data covering a desired surface area of the ocean floor. For example, a single surface vessel may simultaneously tow both a source array and an array of seismic streamers, or different vessels can be used to tow separate source and receiver arrays. Alternatively, a towed source array can be used in conjunction with stationary receivers, for example, an array of ocean-bottom nodes, or with ocean-bottom cables deployed on the seabed.
During operation, acoustic shock waves generated by the source array propagate through the water to penetrate the ocean floor and are reflected from subsurface structures. The reflected acoustic waves are recorded as signals or seismic responses by the receivers, e.g., hydrophones and/or geophones towed behind a vessel or deployed on the ocean floor.
Lateral forces are applied to maintain position and spacing of the seismic sources and other array elements as they are towed behind the vessel. The spacing depends on the number of sources and/or streamer cables that are deployed, and on the spacing between adjacent source and/or receiver components. Typically, a number of source sub-arrays or strings are deployed behind the vessel using a tow rope configuration to spread the sources over lateral distances of approximately ten to one hundred meters or more. Streamer cables are typically deployed over much larger lateral distances, for example, from one hundred meters to a kilometer or more, and may extend for several kilometers behind the tow vessel.
Lateral spacing can be achieved by deploying a paravane or diverter apparatus on a dedicated tow rope arrangement using a spreader or series of individual tether lines to provide the desired spacing between adjacent cables. Positioning devices can also be provided along each streamer cable, in order to maintain depth and/or lateral offset along the cable length.
Generally, paravanes, doors, diverters and similar steering solutions tend to increase drag forces, and require substantial deck area during storage, deployment, and retrieval. Steering response can also be limited, not only by the diverter operating system, but also due to the complex nature or the additional tow ropes, tag lines, and other required elements. As a result, there remains a need for position control systems to provide improved dynamic control with less drag without being subject to other limitations of the existing prior art.
The information included in this Background section of the specification, including any references cited herein and any description or discussion thereof, is included for technical reference purposes only and is not to be regarded subject matter by which the scope of the invention as defined in the claims is to be bound.